The following account of the prior art relates to one of the areas of application of the present application, hearing aids.
Acoustic feedback occurs because the output loudspeaker signal from an audio system providing amplification of a signal picked up by a microphone is partly returned to the microphone via an acoustic coupling through the air or other media. The part of the loudspeaker signal returned to the microphone is then re-amplified by the system before it is re-presented at the loudspeaker, and again returned to the microphone. As this cycle continues, the effect of acoustic feedback becomes audible as artifacts or even worse, howling, when the system becomes unstable. The problem typically appears when the microphone and the loudspeaker are placed closely together, as e.g. in hearing aids. Some other typical situations with feedback problems relate to telephony, public address systems, headsets, audio conference systems, etc.
A particular problem occurs when the coupling conditions of a hearing aid to a user's ear canal is less than optimal, e.g. because the mounting of the hearing aid in the ear canal is less than optimal or because the ear canal changes over time. The former may e.g. occur for people who have difficulty to properly mount a mould of a listening device in the ear canal, and who may need help in mounting the mould and/or deciding on proper mounting. The latter is e.g. the case for children. Because the ears of children grow fast, it is important with a pre-warning by a leakage detector and possibly to lower the gain depending on the detected leakage.
It is known to apply a digital loop gain estimator in a DFC system (dynamic feedback cancellation), and also to realize a digital maximum gain limiter under control of the DFC. This feature is known as a fast online feedback manager. A fast and a slow online feedback managing system are e.g. described in WO 2008/151970 A1.
A method of monitoring a fit of a hearing device is described in WO 2010/049543 A2. The method comprises the steps of estimating a feedback measure reflecting behavior between a receiver and a microphone of the hearing device inserted into an ear, comparing the estimated feedback measure with a predetermined feedback measure, and indicating a misfit once the comparison of the estimated feedback measure with the predetermined feedback measure shows that the hearing device is not properly inserted into the ear canal of the hearing device user.